1. Field of the Invention
The present invention relates to a method of tandem time-of-flight mass spectrometry used in quantitative analysis and quantitative simultaneous analysis of trace compounds and also in structural analysis of sample ions.
2. Description of Related Art
[Time-of-Flight Mass Spectrometer (TOF-MS)]
TOF-MS is an instrument for finding the mass-to-charge ratio of an ion by imparting a given amount of energy to the ion to accelerate it such that it travels and by measuring the time taken until the ion reaches a detector. In TOF-MS, an ion is accelerated with a given pulsed voltage Va. At this time, from the law of conservation of energy, the velocity v of the ion is given by
                                          mv            2                    /          2                =                  qeV          a                                    (        1        )                                v        =                                            2              ⁢              qeV                        m                                              (        2        )            where m is the mass of the ion, q is the electric charge of the ion, and e is the elementary charge. The ion reaches a detector, placed at a given distance of L, in a flight time T. The flight time is given by
                    T        =                              L            /            v                    =                      L            ⁢                                          m                                  2                  ⁢                  qeV                                                                                        (        3        )            TOF-MS is an instrument that separates masses by making use of the fact that the flight time T differs according to different ion mass in as indicated by Eq. (3). One example of linear TOF-MS is shown in FIG. 1. Furthermore, reflectron TOF mass spectrometers capable of providing improved energy focusing and elongating the flight distance by placing a reflectron field between an ion source and a detector are widely used. One example of reflectron TOF-MS is shown in FIG. 2.
[Helical Orbit TOF-MS]
The mass resolution of TOF-MS is defined to be
                              mass          ⁢                                          ⁢          resolution                =                  T                      2            ⁢            Δ            ⁢                                                  ⁢            T                                              (        4        )            where T is the total flight time and ΔT is a peak width. That is, if the peak width ΔT is made constant and the total flight time T can be lengthened, the mass resolution can be improved. However, in the related art linear or reflectron type TOF-MS, increasing the total flight time T (i.e., increasing the total flight distance) will lead directly to an increase in instrumental size. A multi-pass time-of-flight mass spectrometer has been developed to realize high mass resolution while avoiding an increase in instrumental size (see M. Toyoda, D. Okumura, M. Ishihara and I. Katakuse, J. Mass Spectrum., 2003, 38, pp. 1125-1142). This instrument uses four toroidal electric fields each consisting of a combination of a cylindrical electric field and a Matsuda plate. The total fight time T can be lengthened by accomplishing multiple turns in an 8-shaped circulating orbit. In this apparatus, the spatial and temporal spread at the detection surface has been successfully converged up to the first-order term using the initial position, initial angle, and initial kinetic energy.
However, the TOF-MS in which ions revolve many times around a closed trajectory suffers from the problem of overtaking. That is, because ions revolve multiple times round a closed trajectory, lighter ions moving at higher speeds overtake heavier ions moving at smaller speeds. Consequently, the fundamental concept of TOF-MS that ions arrive at the detection surface in turn from the lightest one does not hold.
The spiral-trajectory TOF-MS has been devised to solve this problem. The spiral-trajectory TOF-MS is characterized in that the starting and ending points of a closed trajectory are shifted from the closed trajectory plane in the vertical direction. To achieve this, in one method, ions are made to impinge obliquely from the beginning (see JP-A-2000-243345). In another method, the starting and ending points of the closed trajectory are shifted in the vertical direction using a deflector (see JP-A-2003-86129). In a further method, laminated toroidal electric fields are used (see JP-A-2006-12782).
Another TOF-MS has been devised which is based on a similar concept but in which the trajectory of the multi-turn TOF-MS (see GB2080021) where overtaking occurs is zigzagged (see WO 2005/001878 pamphlet).
[MALDI Method and Delayed Extraction]
Matrix-assisted laser desorption/ionization (MALDI) is available as one type of ion source for TOF-MS. An instrument in which the MALDI method and TOF-MS are combined is referred to as MALDI-TOFMS. In the MALDI method, a sample is mixed and dissolved in a matrix (liquid, crystalline compound, metal powder, or the like) having an absorption band at the used laser light wavelength and solidified. The solidified sample is irradiated with laser light to vaporize or ionize the sample. In a laser-induced ionization process typified by the MALDI method, the initial energy distribution occurring when ions are created is wide. In order to converge the distribution in terms of time, delayed extraction is employed in most cases. This process consists of applying a pulsed voltage after a delay of hundreds of nanoseconds since laser irradiation. The performance of the MALDI-TOFMS has been improved greatly by adoption of the delayed extraction.
A general MALDI ion source and delayed extraction are conceptually illustrated in FIG. 3. A sample is mixed and dissolved in a matrix (such as liquid, crystalline compound, or metal powder) and solidified. The solid sample is placed on a sample plate. To permit the state of the sample to be observed, lens 2, mirror 2′, and a CCD camera are disposed. Laser light is directed at the sample through lens 1 and mirror 1′ to vaporize or ionize the sample. The created ions are accelerated by voltages applied to accelerating electrode 10 and a base electrode (accelerating electrode 20) and introduced into a mass analyzer.
The sequence of TOF measurement steps employing delayed extraction is also illustrated in FIG. 3. First, the accelerating electrode 10 and the sample plate are placed at the same potential of Vs. Then, a signal indicating laser oscillation is received from the laser. After a delay of hundreds of nsec, the potential Vs on the accelerating electrode 10 is quickly varied to V1 to create a potential gradient between the sample plate and accelerating electrode 10, for accelerating the resulting ions. The starting time of measurement of a flight time is synchronized with the rise time of a pulser.
[MS/MS Measurements and TOF/TOF Instrument]
In general mass analysis, ions generated by an ion source are separated according to their m/z by a mass analyzer and detected. The results are represented in the form of a mass spectrum in which m/z values and relative intensity of each ion are graphed. Information obtained at this time is only about masses. This measurement is herein referred to as an MS measurement in contrast with an MS/MS measurement in which certain ions (precursor ions) generated by an ion source are selected by a first stage of mass analyzer (MS1 in FIG. 4), the ions spontaneously fragment or are caused to fragment, and the generated ions (product ions) are mass analyzed by a subsequent stage of mass analyzer (MS2). An instrument enabling this is referred to as an MS/MS instrument.
In MS/MS measurements, the m/z values of precursor ions, the m/z values of product ions generated in plural fragmentation paths, and information about their relative intensity information are obtained and so structural information about the precursor ions can be obtained (FIG. 5). An MS/MS instrument in which two TOF-MS units are connected in tandem is generally known as a TOF-TOF instrument and mainly used in equipment using a MALDI ion source. In a related-art TOF-TOF instrument, a linear TOF-MS unit is adopted as a first TOF-MS unit and a reflectron TOF-MS unit is adopted as a second TOF-MS unit as shown in FIG. 6. An ion gate for selecting precursor ions is disposed between the first and second TOF-MS units. In the case of TOF-TOF, an electric field is normally applied to the ion gate. Under this condition, no ions are allowed to pass. The electric field is de-energized only when precursor ions to be selected pass. In JP-A-2005-302728, the precursor ions spontaneously fragment or are forced to fragment in a collision cell positioned before the reflectron field of the first or second TOF-MS unit.
The performance of the MALDI method has been improved by the employment of a delayed extraction technique. However, the delayed extraction technique has the disadvantage that the focal point differs slightly depending on the m/z value of ion. In the case of TOF-TOF, the focal point is present at the starting point of the second TOF-MS unit. Ion positions are distributed differently according to different m/z values. For example, if ions with some m/z value are focused at the starting point of the second TOF-MS unit, positions are distributed more widely as the m/z value goes away from that m/z value.
That is, if the instrumental conditions are so set that the ion mass resolution is enhanced at some m/z value, the ion mass resolution deteriorates with going away from that m/z value. In order to obtain a product ion spectrum of higher quality, it is important that the precursor ions be focused in the direction of axis of flight at the starting point of the second TOF-MS unit. Therefore, it is necessary to modify the instrumental conditions for each m/z value of precursor ions.
However, modifying the instrumental conditions affects the observed flight time. Consequently, the set value of the ion gate needs to be finely adjusted accordingly. In an instrument capable of providing especially high precursor ion selectivity, if only ions with a quite restricted range of m/z values are selected such as monoisotopic ions within precursor ions, it is quite important to adjust the time setting of the ion gate.